Method and apparatus for auto correcting the distributed temperature sensing system

ABSTRACT

System and method for correcting the potential errors occurring in a fiber optic temperature measurement system are disclosed. In one respect, a dual light sources configuration is provided. The primary light source may illuminate a sensing fiber, and an Anti-Stokes band may be detected. The secondary light source may illuminate a sensing fiber, and a Rayleigh band may be detected, where the Rayleigh band is substantially wide enough to cover the Anti-Stokes band of the primary light source. A ratio between these Anti-Stokes and the Rayleigh bands may be used to measure the temperature and undesired errors due to the perturbations falling on the sensing fiber is continuously corrected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 14/182,533filed on Feb. 18, 2014, now U.S. Pat. No. 9,645,018. Priority is claimedfrom U.S. Provisional Patent Application Ser. No. 61/766,639, filed Feb.19, 2013, which is hereby incorporated by reference in its entirety.

BACKGROUND

The present invention relates generally to temperature sensing. Moreparticularly, the present disclosure relates to system for automaticallycorrecting the temperature measurement in a distributed system.

Optical fibers have been used mainly for optical communication systemsfor decades. Recently, optical fiber sensing technologies have grownrapidly due to their advantages over conventional sensing devices. Theoptical fiber sensors can handle much higher signal bandwidth andtemperature ranges, are immune to electromagnetic interference noises,provide safe operation (no generation of electric sparks whichoccasionally induce catastrophic fire incidents), and require a mucheasier installation process. But the most prominent feature is theircapability of true distributed parameter measurement i.e., the extendedranges' temperature monitoring can be covered with a single fiber opticcable. Utilizing this distributed technology, temperature or otherparameter's profile along significant distances can be monitoredcontinuously without any electric wire involvement. Many temperaturedata points can be processed along over 10 kilometers within a shorttime. The resultant distributed measurement is equivalent to theperformances of thousands of ‘point temperature’ sensors, which usuallyare susceptible to electric noises, occasional fire incidents due towire shorted circuits and complicated installation—costly and take longperiods of time. Thus the fiber optic distributed sensing systemprovides for long range temperature measurement with reliableperformance, safe operation and economic installation.

When a laser light with a center wavelength (1) is injected into a fibercable, most of the light is transmitted, but small portions of incidentlights are scattered backward or forward along the fiber. Thesescattered lights are categorized into three unique spectralbands—Rayleigh, Brillouin and Raman bands. For the measurement ofdistributed temperature, typically few components such as Rayleigh (1R,insensitive to temperature), Stokes (1S, longer than 1R and lesssensitive to temperature) and anti-Stokes (1AS, shorter than 1R and mostsensitive to temperature) of Raman band have been used. These opticalsignals may be separated by optical filters (or other wavelengthselecting devices) and are received by the photo detectors to convertlight to electrical signals. The temperature can be calculated by theratio of temperature sensitive anti-Stokes to less sensitive Stokes ortemperature insensitive Rayleigh components.

To obtain the temperature profile along the distance, two processingmethods—time domain processing approach and frequency domain approachhave been applied conventionally. The time domain method (orOTDR—Optical Time Domain Reflectometry) uses pulsed light source, andthe location of the temperature is identified by the calculation of thepulse's round trip time to the distance under test. The frequency method(OFDR—Optical Frequency Domain Reflectometry) uses a modulated lasersource, and the position can be calculated by applying the inverseFourier transformation of the sensing fiber's transfer function or thefrequency response. The OTDR method is a one step process and providesquicker response time but needs high pulse energy to obtain high SNR(signal to noise ratio). OFDR method takes longer process time becauseit is a two step process (convert from frequency domain to time domain),but higher light power can be applied to increase the SNR. Also randompulse coded (multiple pulses) based on time domain methods can beapplied. All approaches have their pros and cons, and the selection maydepend on the application.

Even though the DTS has been widely applied to many areas so far, twocritical issues should be handled properly for reliable long termmeasurement. The first issue is the inherent characteristics ofanti-Stokes band, which are not only sensitive to temperature variationbut also to physical perturbations, which induce the attenuation of thelight transmitting along the fiber. This ambiguity can be corrected byusing another reference light source(s), whose Rayleigh band 12R (onlysensitive to the attenuation only) is located in Stokes or anti-Stokesband of the first light source, 11AS. The second issue is the DAF(Differential Attenuation Factor) due to the material characteristics ofoptical fiber. For given external perturbations, all transmitting lightin the fiber cable including wavelengths of 1R, 1S and 1AS experiencedifferent attenuations. Typically, the shorter the operating 1 is, thehigher the attenuations are. These phenomena should be considered andhandled properly for reliable temperature calculation by the ratio of1AS to 1S or 1AS to 1R bands. Typically this effect is corrected justafter the sensing fiber is deployed. But in many real application cases,various perturbations are applied or generated unintentionally to thesection of sensing fiber cable times after deployment. Theseenvironmental effects are called the ‘darkening’ in a local section orsections of the fiber and induce different attenuation values to each 1,resulting in erroneous temperature profile. Those effects may be:radiation related darkening, hydrogen gas related darkening andadditional stresses applied on the arbitrary length of the fiber cable.Therefore this phenomenon should be corrected automatically (or selfcorrected) and continuously for accurate measurement. To correct DAFrelated errors, the DE (Dual End) method has been applied, which isimplemented by two channel configuration (Ch1 and Ch2 are used for asensing) and utilizes common loss compensation method between these 2channels. After the primary laser light is injected to Ch1 and Ch2consecutively by a switching device, the detected signal of Ch2 (oreither one) is rotated in mirror image and subtracted (commonattenuations) from the other channel, removing the error. The issue ofthis method is the requirement of two channels, which requires twice themeasurement time, double the length of sensing fiber and twice theoptical budget. Also DE configuration is not permissible for somecritical applications. Finally, the temperature effect of the Stokesband is subtracted for high temperature measurement because the Stokesband is still dependent on temperature even though it is not assensitive as the AS band.

The major trend of temperature calculation has been utilizing the ratio(R) of AS to S band because their light intensities (IS and I AS) arecomparable to each other. It is expressed mathematically as:

${R(T)} = {\frac{I_{AS}}{I_{S}} = {\left( \frac{\lambda_{S}}{\lambda_{AS}} \right)^{4}{\exp\left( {- \frac{hcv}{k_{B}T}} \right)}}}$where h, c, k_(B), v and T are Plank constant, the speed of light,Boltzmann constant, Raman wave number shift and absolute temperature indegrees K, respectively. A similar equation can be applied for otherratios such as AS to R, both are in same or different bands. Thecorrection method using AS/R ratio instead of AS/S ratio is moreeffective in terms of the amplitude of DAF (approximately half of AS/Scase by an interpolation method) and the smallest temperature dependencyof R band. But the amplitude of R is usually a few orders larger than ASband, and it is necessary to correct DAF continuously during all themeasurement periods. The challenging job is the process of continuousin-situ correction for DAF and the subtraction of the temperaturedependency of S band in high temperature application.

The disclosure in U.S. Pat. No. 7,126,680 described the correction ofattenuation related effect of 11S and 11AS by two independent extrareference light sources 12 and 13 (total 3 light sources are required).In this case, several conditions need to be considered to be aneffective correction—1) two extra light sources center 1 and theirbandwidths should be identical and kept stable. 2) Scattered signalssuch as anti-Stokes or Stokes bands are much wider than extra lasersources' Rayleigh bands. Also the intensities of two reference lightsources should be comparable and compensated to suppress the errorrelated to the momentary fluctuations.

Finally, the temperature effect of the Stokes band is subtracted. Theother correction algorithm using two light sources was disclosed in U.S.Pat. No. 4,767,219. In this disclosure, two light sources need to beselected to satisfy the condition that 1/11=1/12=1/v where v is Ramanshift in wave number. Another scheme was disclosed in U.S. Pat. No.7,628,531 by selecting two light sources such as 11AS=12S. For these twolight source cases, DAF issue can be corrected automatically because twobands are located in the same center wavelength. But their bandwidthsize as well as center wavelengths should be precisely matched and keptstable to ensure the effective correction continues. Also theintensities of secondary light sources should be stabilized to reducetemperature error related to the momentary fluctuations of the source.Last, the nature of temperature dependency of the S band should besubtracted. Because of various issues mentioned so far, the idealcorrection method is to use the ratio between AS and R, which has thesame wavelength bands with comparable amplitude. For continuouscorrection approaches, the following one source, two sources and 3sources method have been disclosed as described below.

The idea using AS band to R band ratio with three sources was disclosedin U.S. Pat. No. 7,284,904. In this scheme, 3 lights 11 (primary light),1-1 and 11LO (two auxiliary lights) were proposed for the scheme. 11LOis the light source with same wavelength as 11 but has lower opticalpower and 1-1 source is same wavelength as 11AS generated by 11. DAFcorrection was claimed by two auxiliary spectral bands 11LO and 1-1 bynormalization and interpolation. But interpolation is an indirectcorrection method, and there may be a practical implementation limit tomatch and keep the wavelengths stable. Another auto correction methodbetween the anti-Stoke band and R band with one light source wasdisclosed in U.S. Pat. No. 7,350,972. To generate R band match to ASband, a semiconductor laser was operated in two modes, from a laser mode(stimulated emission) to an LED mode (spontaneous emission) by applyingthe driving current under threshold level consecutively. In the LEDmode, the spectral width is widened but the light intensity is alsosignificantly decreased under impractical level. This simpleauto-correction scheme has two concerns: 1) LED mode's spectral bandshould be wide enough to cover both whole AS band and its original Rband, which is separated around 50 to 100 nm in a single side of theband (depends on operating 1, 800 nm to 1550 nm) from R band, and 2) itsoutput power of LED mode should be high enough to be a practicalimplementation.

SUMMARY

In present disclosure, a novel automatic DAF correction method, whichcan be applied to all of OTDR, OFDR and multiple pulse coded DTS schemesis introduced. This method is based on two light sources scheme, whichsatisfy the following three conditions: 1) the center wavelength of thesecond light sources 12R is selected to be located near the middle offirst AS and R bands (i.e., (11AS+11R)/2), 2) its spectral bandwidth iswider enough to cover the entire AS and R band of 11, and 3) the secondlight source has an output high enough to be a practical implementation.The light source classified as SLD (Superluminescent Diode), which hasthe combined characteristics of a laser and a LED, or other broadbandsource can be used for this purpose. SLD sources are commerciallyavailable from manufacturers such as Superlum, Q-Photonics, Thorlabs,Dense Light and Exalos. These manufacturers have various product linesof wavelengths and output level. Then temperature calculation and theauto correction is made through the ratio of same bands signals i.e.,11AS A2R(1As), where 12R,1AS, is the Rayleigh band of the second source,whose spectra are overlapped by the first source's 11AS. The other bandof the second light source, 12R, R1, 2, is used for the compensation ofinstantaneous variations of the band 12R(1AS) as illustrated in FIG. 1.

This new disclosure can provide a reliable and accurate temperaturecalculation by 1) auto correcting the DAF issue and 2) wide temperaturerange application without the temperature effect subtraction from theStokes band because this scheme utilizes the ratio of temperaturesensitive Anti-Stokes and temperature insensitive Rayleigh signals, bothare located in same spectral bands and 3) continuously compensating thefluctuations in the light source output intensities.

This scheme utilizes two light sources, which have the following threematching conditions. First, the center wavelength of the secondary lightsource is located at the middle of primary light's AS band plus R band.Second, the spectral bandwidth (total R bandwidth) of the secondarysource is wide enough to cover the primary light's AS band plus R band.Third, the amplitude of the secondary source needs be comparable (insimilar range) to the primary light's AS intensity. Commerciallyavailable broadband and high power devices such as SLD sources can beused for these implementations.

The primary and secondary light sources are selected alternately andconsecutively by an optical switch or by applying a group of electricalpulses following their time sequences.

Temperature calculation is made by the ratio of AS band of the primarylight and R band of the secondary light, both located in the samespectral band as described above. In this case, DAF issue isautomatically corrected.

The instantaneous output fluctuations of both light sources aremonitored continuously by the external light detecting device (such as aphoto diode). The transmitted light is selected by the coupler toseparate most light to the sensing fiber and a small portion to thelight detecting device. The internal light detecting device mounted inthe light source can be used for the alternative.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the disclosed subjectmatter will be set forth in any claims that are filed later. Thedisclosed subject matter itself, however, as well as a preferred mode ofuse, further objectives, and advantages thereof, will best be understoodby reference to the following detailed description of an illustrativeembodiment when read in conjunction with the accompanying drawings,wherein:

FIG. 1 shows a spectral requirement of the disclosure with filteroutputs.

FIG. 2 shows a system configuration of a correction system with aswitching device.

FIG. 3 shows a system configuration of a correction system with anelectrical scheme.

FIG. 4 shows a system configuration of a correction system with externalphoto detectors.

FIG. 5 shows a system configuration of a correction system with internalphoto detectors mounted inside of the light sources.

FIG. 6 shows a system configuration of a correction system with anoptical circulator.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows the spectral requirements to select the secondary lightsource. The top figure illustrates 3 spectral bands 100 of the first (orprimary) light source, anti-Stokes, Rayleigh and Stokes respectively.The bottom figure shows the secondary source's spectral components. Thecenter wavelength of the secondary is selected to be located in themiddle of anti-Stokes 101 and Rayleigh bands 102 of the primary sourceand are wide enough to cover both spectral bands. A three port (oneinput and two outputs) wavelength selector is used to select two bands,.lamda..sub.1AS, .lamda..sub.1R, or .lamda..sub.2R,1AS and.lamda..sub.2R,1R or .lamda..sub.2,1R.

FIG. 2 shows the embodiment of the system diagram. Two light sources,the primary source 103, L1 (conventional laser type) and the secondary104, L2 (a SLD light source) chosen as the conditions mentioned in theprevious page is consecutively selected by an optical switch, 105 toselect L1 first (at time=t1) and inject the light to the sensing fiberthrough the coupler, 106. Its backscattered .lamda..sub.1AS, and.lamda..sub.1R optical signals from the fiber (including the reference,107, and the sensing portion, 108), are collected at the two outputports, 101 and 102, of the optical filter (or other type of wavelengthselector) simultaneously. Then selected two light signals are guided toeach photodetector (PD), 109, to convert optical signal to electricalsignal for data acquisition process. After finishing t1 process, attime=t2, the light source L2 is selected and the portion ofbackscattered light R band 101, which has the same spectral band withL1's AS band and the other part of R band 102 are collectedsimultaneously by the same photodiodes, 109. Then temperature iscalculated from the ratio of AS and R, .lamda..sub.1As and.lamda..sub.2R,1AS. Instead of 2 PDs, a single PD can be used byapplying an optical switch.

FIG. 3 shows the system diagram using an electrical triggering method toselect the primary and the secondary light sources by a programmedscheme instead of the optical method described in FIG. 2. The group ofelectric pulses, 110, (depends on processing time) is selectedconsecutively to modulate each light source and supplied to thereference and sensing fiber through a fiber coupler 111. For thisembodiment, one group of consecutive pulses is generated and selectedand applied to each source by an electric switching method.

FIG. 4 shows the embodiment diagram to compensate the instantaneouslight output fluctuation by external photo-detectors. Two externalphoto-detectors, 121 and 121, are dedicated for each light sourcethrough the optical couplers, 123 and 124, to split the output to thephoto-detector and to the sensing fiber. By selecting the couplingratio, such as 1 to 99, most of the light (portion of 99%) can betransferred to the sensing fiber for the efficiency of limited lightintensity. By this scheme, the light transmission and compensation canbe made simultaneously. After the fluctuation is determined by 121 and122, this amount is used to compensate the auto-corrected temperaturecalculation continuously at 150 (a signal processor).

FIG. 5 shows the embodiment diagram to compensate the instantaneouslight output fluctuation by internal photo-detectors. Two internallymounted power monitoring photodiodes, 131 and 132, inside of the lightsources are utilized for this embodiment. This is a popular,commercially available configuration to monitor the light outputvariation. By this scheme, the light transmission and compensation canbe made simultaneously without any power loss. After the fluctuation isdetermined by 131 and 132, this amount is used to compensate thetemperature calculation at 150.

FIG. 6 shows the embodiment diagram, which uses an optical circulator,133, instead of fiber optic coupler. This embodiment can savesignificant optical power loss because of lower insertion loss of thecirculator compared to 50:50 fiber couplers, which couples only 50% ofoptical power for both transmission and reflection.

What is claimed is:
 1. A system for automatically correcting temperature measurement comprising: a fiber optic cable; a first light source optically connected to said fiber optic cable, said first light source producing a first light signal in said fiber optic cable; a second light source optically connected to said fiber optic cable, said second light source producing in said fiber optic cable a second light signal comprising a center wavelength located in the middle of an Anti-Stokes band plus a Rayleigh band of the first light signal, a spectral bandwidth broader than said first light signal's Anti-Stokes band plus Rayleigh band, and an amplitude comparable to an Anti-Stokes intensity of said first light signal; and a light detecting device optically connected to said fiber optic cable for monitoring the first and second light signals in the fiber optic cable produce in the fiber optic cable.
 2. The system of claim 1, further comprising one or more optical filters optically connected to said fiber optic cable, wherein said one or more optical filters filter said Anti-Stokes and Rayleigh signals.
 3. The system of claim 1, wherein said first light source is a laser.
 4. The system of claim 1, further comprising: a coupler optically connected to the system such that said coupler is capable of diverting a portion of said first and second light signals from said fiber optic cable to said light detecting device.
 5. The system of claim 1, wherein said light detecting device is a photo diode.
 6. The system of claim 1, wherein said light detecting device is external to said first and second light sources.
 7. The system of claim 1, wherein said light detecting device is internal to said first and second light sources. 